Microsoft Excel has some nice pre-defined styles to easily and quickly highlight a group of cells, columns, or rows.
However, if you’re using these to mark your location in a sheet as you’re entering data or for some other temporary reason, a bug in Microsoft Excel 2016 (Apple Mac version) will cause the date formatting to be lost in any columns that were defined to be formatted as dates. So, the dates will be converted into a scientific notation equivalent number.
If this happens, you need to highlight the relevant column and then apply the cell formatting you’d prefer.
To avoid this problem, don’t use the predefined Excel styles. Instead, use the Fill color option found on the Home ribbon just to the right of bold/italic/underline. This can be applied and then removed without impacting the cell formatting.